OSB roof sheathing is the most popular choice for sheathing today. It's made of wood chips and strips that are compressed together to form a flat 7/16" sheet.
OSB stands for Oriented Strand Board, and it is an engineered wood panel similar to plywood - the major difference here is how it's made. OSB is made up of wood strands 8-15 cm long, that are then mixed with resin and interleaved together in thick sheets.
The sheets are bonded together using heat and pressure. Since OSB is produced in large, continuous mats, it's a solid-panel product of consistent quality with few voids or gaps.